Since defibrillation is probabilistic and shock strength dependent, shocks of the same strength sometimes succeed and other times fail (Davy J M et al, Am. Heart J. 1987;113:77-84). It would be very useful if there were a way to predict defibrillation outcome soon after the shock so that the rescue shock could be delivered quickly with less time spent in ventricular fibrillation (VF) (Weaver W D et al, Journal of the American College of Cardiology 1986;7:752-7; Echt D S et al, Pacing and Clin. Electrophys. 1988;11:1315-1323).
Several studies have investigated activation before and after successful and failed shocks to try to predict defibrillation outcome. Those studies that used different shock strengths focused on the observation that the postshock interval (i.e. the interval between the shock and the first postshock activation) can be used to predict the shock outcome. In these studies the shock strengths for successful episodes were generally higher than for the failed episodes. Other studies that used the same shock strength for both successful and failed episodes did not find this relationship. Despite this controversy, all studies agree that, following shocks near the defibrillation threshold, a few organized ectopic cycles of activation occur prior to degeneration into VF for failed shocks or return to sinus rhythm for successful shocks. (Chen P -S, et al.: J. Clin. Invest. 1986;77:810-823; Shibata N et al: Am. J Physiol. 1988;255: H902-H909; Tovar O H and Jones J L: Am. J Physiol. 1997;272: H1011-H1019; Zhou X, et al.: Circ. Res. 1993;72:145-160; Usui M, et al: J Cardiovasc. Electrophysiol. 1996;7:322-334). Hence, there remains a need for new ways to predict defibrillation outcome promptly after the delivery of a cardiac therapy pulse.